The present invention relates to a variable cycle propulsion system for a supersonic airplane making it possible both to obtain high thrust and a large bypass ratio for takeoff, landing, and subsonic cruising flight so as to greatly reduce and improve specific consumption during these stages of flight, and also to obtain a high exhaust speed adapted to supersonic cruising flight.
More particularly, the invention relates to a propulsion system having auxiliary fans separate from the engines and adapted to subsonic flight (in terms both of noise and fuel consumption).
In another aspect of the invention, it also relates to a method of operating said variable cycle propulsion system.
When designing a supersonic commercial airplane, there arises the particular problem of keeping airplane engine noise low during takeoff, climbing, and landing. In order to be certified, all airplanes must nowadays comply with low-noise regulations for takeoff and landing.
Furthermore, supersonic airplane engines must also satisfy requirements of low engine pod drag during supersonic cruising, low specific fuel consumption while overflying inhabited areas at subsonic cruising speed, and reduced emissions of nitrogen oxide pollution close to the ozone layer at high altitude.
In order to satisfy those various requirements, engine manufacturers have proposed a variable cycle engine for supersonic airplane propulsion. Typically, that type of engine takes up two different configurations: one for subsonic cruising flight, takeoff, and landing; and the other for supersonic cruising flight, given the degree of incompatibility between those two stages of flight in terms of engine operation.
The requirement for low engine noise during takeoff and landing implies, in particular, that gas should be ejected at low speed during takeoff and landing and while cruising at subsonic speed, and that is in contradiction with being able to cruise at supersonic speed, which requires gas to be ejected at high speed.
Noise level depends on gas exhaust speed, and to reduce noise to an acceptable level, exhaust speed must nowadays be less than 400 meters per second (m/s), which corresponds to a threshold of 103 decibels (dB) (with new regulations reducing this to 300 m/s or 90 dB as from the year 2006). Such an exhaust speed thus implies an engine having low specific thrust, which corresponds to a large bypass ratio, i.e. a high level of drag when cruising at supersonic speed.
Thus, the variable cycle engines proposed by manufacturers seek to combine low engine noise during takeoff and landing, low specific fuel consumption during subsonic cruising, and high specific thrust during high altitude supersonic cruising.
Various variable cycle engine designs are known, however varying the bypass ratio of such designs does not enable good optimization both in the subsonic configuration and in the supersonic configuration.
Adopting a gas exhaust speed equal to or less than 400 m/s requires an engine pod of large diameter, and all variable cycle engines known at present thus require a pod front section that is greater than the optimum section for supersonic cruising flight.
For example, U.S. Pat. No. 5,529,263 discloses a supersonic airplane having a propulsion assembly for takeoff, landing, and subsonic cruising flight, and two engines adapted for supersonic cruising flight. The propulsion assembly is constituted by retractable high-bypass ratio booster turbojets, which presents numerous drawbacks, in particular concerning bulk and weight for the airplane.